Since at least the early 1960's, liquid crystalline polymers have been used to produce high strength fibers. Well known examples of these types of fibers include aramid fibers made from highly-oriented rod4ike polymers of poly(paraphenylene terephthalamide), well known as KEVLAR.RTM. aramid fibers commercially available from E. I. du Pont de Nemours and Company, Wilmington, Del. or Twaron fibers, commercially available from Akzo Nobel NV, Netherlands. These aramid fibers provide exceptional tenacity and a high tensile modulus. Breaking strengths of 2.3-3.4 gigapascals (GPa) with a modulus of 55-143 GPa are typical for these fibers. This, combined with their low specific gravity and thermal stability, has resulted in improved performance in many structural applications such as aircraft, boats, sporting goods, missiles and armor. However, a major drawback of these types of fibers has been their relatively poor fiexural rigidity and compressive properties. Fibers yield at low values of stress on the order of 400 megapascals (MPa) with the formation of kink bands.
In order to alleviate this difficulty, much effort has gone into attempts to cross-link the polymer in the filaments, but to date there has been little success. Another approach has been to coat the fiber with a sufficiently high modulus material, to, in effect, "girdle" the filament and prevent buckling. Early work by McGarry et al., SAMPE Quarterly, p. 35, July 1992, demonstrated the effectiveness of this approach with vapor deposited alumina coatings. Recently, enhanced properties have been reported for the microwave plasma assisted organometaUic deposition of TiN coatings on KEVLAR.RTM. aramid fibers.
An alternative coating for KEVLAR.RTM. aramid fibers with potential for improving the mechanical properties of the fibers is "diamond-like-carbon" (DLC). DLC is a smooth amorphous solid made up of a highly cross-linked carbon network with a substantial degree of sp.sup.3 bonding. This sp.sup.3 bonding results in mechanical properties approaching that of diamond itself. The fraction of sp.sup.3 bonding can vary from about 10 percent to about 90 percent depending upon the deposition process and the processing conditions, yielding films with properties ranging from polymer-like to diamond-like. Typical values of modulus for hard coatings are in the range of 20 to 177 GPa. This, combined with low density, low coefficient of friction, high hardness and low deposition temperatures, makes for ideal material for coating aramid fibers.
Yet, the coating of non-conductive materials such as aramids is not straightforward. Previously, the deposition of diamond-like carbon onto KEVLAR.RTM. aramid fibers has been accomplished by initially pre-coating the fiber with a thin nickel layer to confer conductivity. It is desirable to coat the non-conductive fiber, e.g., the KEVLAR.RTM. aramid fiber, without the need for any intermediate metal layer.
It is an object of the present invention to provide an apparatus for coating a non-conductive fiber, especially an aramid fiber, such as KEVLAR.RTM. aramid fiber, with diamond-like-carbon.
It is a further object of the invention to provide a process for coating a non-conductive fiber, especially an aramid fiber, such as KEVLAR.RTM. aramid fiber, with diamond-like carbon. Other objects and advantages of the present invention will become apparent to those skilled in the art upon reference to the attached figures and to the detailed description of the invention which hereinafter follows.